pg_start_backup: file has vanished from pg_subtrans/

Hi,

While backing up some postgresql-8.4.2 instances using
pg_start_backup()/pg_stop_backup()

We noticed that sometimes rsync complains:

file has vanished: "............./data/pg_subtrans/3A37"
rsync warning: some files vanished before they could be transferred (code 24) at main.c(1039) [sender=3.0.6

We already have excluded pgsql_tmp/ pg_xlog/ and pg_stat_tmp/ so I was
tempted to exclude pg_subtrans/ as well. However some googling seems to
suggest that this might invalidate the backup:

https://groups.google.com/forum/#!msg/pgbarman/zQxg6pOXkZ8/AmTmVtUA3dEJ

I am not sure what to believe, either
- my backup was invalid and I have to take measures to prevent this
(and file a bug report)
- I can ignore this *and* can safely exclude pg_trans/ from my backups.
- there is some other workaround...

--
Leo Baltus